Hydraulic power brake booster

ABSTRACT

A hydraulic power brake booster which utilizes open center valve means positioned concentrically of the booster piston in a manner to provide follow up action therebetween. The valve is equipped with a trim adjustment for regulating flow between the ports and to the respective ends of the booster piston. Operator-operated means including lever means is interconnected between the valve means and booster piston means to establish one ratio of lever to piston movement for power actuation and a different, higher ratio, of lever to piston movement for no-power.

United States Patent [191 Brown, J r.

11 3,733,966 1 May 22, 1973 [54] HYDRAULIC POWER BRAKE BOOSTER ArthurKing Brown, Jr., South Bend, Ind.

[73] Assignee: The Bendix Corporation, Bend, Ind.

[22] Filed: Mar. 24, 1971 [21] Appl. No.: 127,684

[75] Inventor:

South [52] US. Cl ..91/372, 91/378, 91/391 R,

[51] int. Cl......F15b 13/16, F15b 13/10, Fl5b 15/17 [58] Field ofSearch ..91/39l R, 391 A, 91/165, 417, 372, 378

[56] References Cited UNITED STATES PATENTS 2,755,891 7/1956 Levell eta1. ..9l/391 A 9/1965 Schultz ..91/391A 10/1971 Meyers ..9l/391R PrimaryExaminer-Paul E. Maslousky Attorney-Ken C. Decker and Plante, HartzSmith & Thompson [5 7 ABSTRACT 9 Claims, 5 Drawing Figures as 68 46 9'5[l2 as 92 54 94 9 /l6 /24 lZZ 52 83 loo 2 42 5o 24 127 PATENTED WY 221975 3. 7 33 9 6 6 sum 1 0F 2 INVENTOR.

= R OWN JR.

PATENTED 3.733.966

I N VEN TOR.

%C'QM/ 4 ARTHUR K. BROWN JR.

BACKGROUND OF THE INVENTION The present invention in general relates toa fluid operated booster for converting a manual input force into anamplified output force. More particularly the invention concerns such afluid operated booster wherein valve means and piston means are sointerconnected that in the event of a failure of the fluid pressure thebooster piston is operated manually by an applying force not toodifferent from the force required to operate a conventional non-powerbraking system. The booster is of particular utility in associated witha master cylinder of a hydraulic braking system and for convenience willbe shown in such relationship.

Reference is made to U. S. Pat. Nos. 2,766,732; 3,119,235 and 3,267,663,assigned to the common assignee for this invention, as showing fluidboosters of the aforementioned general character. The present inventionrelates to improvements on the boosters of such patents.

SUMMARY OF THE INVENTION The principal object of the invention is toprovide a fluid operated booster of the foregoing type having means formanually operating the booster piston at a higher ratio of foot pedal topiston movement upon failure of the fluid pressure than that ratioemployed to operate the piston with fluid pressure available. I

Another object of the invention is to provide trim adjustment means forthe open center valve means utilized in the booster to obtain the properdifferential pressure acting on the respective ends of, the boosterpiston for returning it to the unapplied or released position.

A still further object of the invention is to provide a fluid operatedbooster of the foregoing construction requiring only two hoseconnections, one to the pump and one to the steering gear.

An important object of the invention resides in the provision of a fluidpressure system for a vehicle having an open center valve steering gearconnected in series flow relationship with a brake boost mechanism, thecombina-tion of which includes valve means for dividing system pressurebetween said steering gear and said mechanism.

The above and other important objects and features of the invention willbe apparent from the following description of the mechanism taken inconnection with the accompanying drawings which form a part of thisspecification.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, partially in section, of ahydraulic power booster constructed in accordance with the inventionwhich is disclosed in association with other elements of the vehiclehydraulic system illustrated sche- FIG. 5 is a fragmentary sectionshowing the chamfer on the spool valve.

DETAIL DESCRIPTION Referring to FIG. 1 it will be noted that referencenumeral 10 designates a fluid operated boost mechanism installed in avehicle hydraulic system having an engine driven pump 12 and a steeringgear 14 operatively connected to the vehicle wheels, not shown, in amanner well known to those skilled in the art. The boost mechanism 10,which may be termed more specifically a hydraulic brake booster sinceits principal use is in conjunction with a brake master cylinder, notshown, comprises a housing means 16 provided with cylinder means 18including end closure members 20 and 22 held in their respectivelocations by snap rings 24 secured in grooves 26 in the cylinder means.The end closures 20 and 22 are equipped with the necessary sealingelements 27 to prevent the escape of fluid from said cylinder means toatmosphere. Inlet and outlet ports 28 and 30, respectively, are formedin the housing means at axially spaced locations in communication withsaid cylinder means. The inlet port 28 is connected to the outlet of thepump 12 by conduit 32. The outlet port 30 is in fluid communication withthe steering gear 14 via pipe line 34. Fluid return from the steeringgear 14 is connected to the inlet of the pump 12 by conduit 36, thusproviding for recirculation of the fluid. The steering gear 14 per sewill not be described in detail since it is not a part of the invention.However, it should be mentioned that the gear is of the conventionalopen center valve construction as shown in U. S. Pat. No. 1,790,620.During non-steering conditions the valve allows fluid to freelycirculate therethrough. During steering conditions the amount of fluidpassing through the steering gear valve is determined by the severity ofthe steering requirement.

The boost mechanism 10 is connected in series flow relationship to thesteering gear 14, thus requiring only two connections in the housingmeans 16, one to the inlet port 28 and the other to the exhaust port 30.With such an arrangement the boost mechanism 10 must share systempressure with the steering gear 14 at times of simultaneous operation.To insure sharing, a pressure relief valve 33 is connected between theinlet port 28 and exhaust port 30. The setting of the valve 33 isdetermined by the minimum operating pressure requirements of thebooster.

Reciprocally positioned within the cylinder means 18 is piston means 38,which together with said cylinder means divides the latter into twochambers 40 and 42,

located at the respective ends of piston means. The piston meansincludes a piston member 44 provided with lands 46, 48 and 50. Acircumferential recess or cavity 52 is formed between the lands 46 and48 and circumferential recess or cavity 54 is formed between the lands48 and 50. Each of the lands 46, 48 and 50 is equipped with appropriatesealing means 56 to maintain the desired pressures in the chambers 40and 42 and cavities 52 and 54. The cavities 52 and 54 are of such axiallengths as to maintain continuous communication with the exhaust port 30and the inlet port 28, respectively, during full piston stroke in thecylinder means 18. The piston means further includes a tubular member 58secured in an axially extending central bore 60 of the piston member 44by a locking pin 62 located in a transverse bore 64 of the piston 44, asbest shown in FIG. 4. The transverse bore 64 opens into the central bore60 so that a portion 65 of the pin 62 extends into an annular groove 66of the tubular member 58. The groove 66 is slightly over-size withrespect to the pin 62 to allow for misalignment between the pin andgroove during assembly of the parts. A snap ring 68 fits into a groove70 of the member 58 to further secure the piston 44 to the tubularmember 58 for driving engagement. An O-ring 71 provides sealing betweenthe tubu lar member 58 and the piston 44. End 72 of the member 58projects through an opening 74 of member 20. A packing and sealing gland76 is carried in circumferential groove 78 of the end closure 20 tothereby seal the chamber 40 from leakage. A force transmitting rod 80has one end threadedly secured to the member 58 and its other endconnected to a hydraulic brake master cylinder, not shown, for actuatingthe same, in a manner well known to those skilled in the art.

Valve means 82 of the open center type controls the operation of theboost mechanism 10. The valve means 82 includes a spool member 84 whichis concentrically located in the bore 60 of the piston member 44 forcontrolling fluid pressure in the chambers 40 and 42 of the cylindermeans. The valve spool 84 is formed with annular channels 86, 88 and 90which cooperate with annular channels 92 and 94 formed in the centralbore 60 for regulating fluid flow between the ports 28 and 30 and thetwo chambers 40 and 42. Inlet fluid pressure is communicated from therecess 54 to channel 88 through a radial passage 96. A longitudinalpassage 98 in the piston wall intersects a radial passage 100 also inthe piston wall to establish a fluid connection between the channel 94and the chamber 40, at the left end of the piston as viewed in FIG. 1.Communication between the chamber 42, at the right end of the piston asviewed in FIG. 1, and the channel 92 is through longitudinal passage 102and radial passages 104 and 106 in the spool valve 84. Recess 52, incontinuous communication with the exhaust port 30, is in fluidcommunication with channels 86 and 90 through radial passages 107 and108 and a longitudinal passage 110 which intersects said radialpassages. It is to be noted that the effective areas of the oppositeends of the piston member 44 exposed to chambers 40 and 42 are equal.This becomes apparent when it is realized that the outside diameter ofmember 58 is the same as the outside diameter of spool valve 84. Withparticular reference to FIG. 5, it is seen that theland opposite channel94 is chamfered at 95 so that maximum shifting of the spool to the leftwill not reduce fluid flow at the exhaust port 30 to a levelinsufficient to operate the steering gear 14. The chamfer provides abypass orifice 97 which insures continuous flow from port 30.

In order to establish the desired fluid pressure levels in the variouschannels and chambers of the boost mechanism, the valve means 82 isequipped with adjusting means 112 for trimming the valve as needed. Theadjusting means 112 includes a screw 113 slidably received in passage114 of the tubular member 58 so that head 115 of the screw engages ashoulder 116 to permit axial movement or trimming of the spool bythreading end 118 of the screw in or out of the threaded bore 120 of thespool. The passage 114 is counterbored at 122 for receiving a valvecentering spring 123 through which the screw 113 passes. The spring isinterposed between an end 121 of the valve spool and a shoulder 124located at the bottom of the counterbore. The means 112 adjustablyselects the valve spool position in which the back pressure in chamber40 is higher than the back pressure in chamber 42 to assist in returningthe piston 44 to the brake booster released position. The spring 123retains the spool in its adjusted position but allows for axial displacement of the spool in response to a brake application. The valvespool 84 is threadedly secured to a hollow extension member 126 whichpasses through an opening 127 of the end closure 22. A packing gland128'located in the end member 22 seals the chamber 42 from fluid leakagebetween the closure member and the valve spool extension 126. The valvemeans 82 and piston means 38 are operatively connected tooperatoroperated means 130.

The operator-operated means 130 is a dual ratio mechanism, establishinga low pedal or lever ratio for fluid actuation of said piston 44 and ahigher pedal or lever ratio for solely manual actuation of said piston.The means 130 includes a link or plunger 132 slidably carried in bore133 of the hollow extension member 126. The inner end of the plunger 132is provided with a rubber cup 134 which seals chamber 136, incommunication with the passage 102 of the spool 84, from fluid leakagebetween the plunger and the bore 133. It

is to be noted that the reaction area 135 formed on the I annulus at theinner end of the extension 126 is less than the reaction area 139developed across the diameter of the bore 133 in chamber 136. Thisrelationship of areas is essential to actuation of the valve means 82.The plunger 132 is formed with a slot 137 through which a pin 138passes. The pin 138 is transversely positioned across the centralportion of the bore 133 with the ends of the pin being secured in thewalls of the extension 126. The pin 138 not only retains the link 132 inthe bore 133 but it controls its travel. The end of the plunger 132opposite the cup 134 is provided with a rounded or spherical end portion140 over which a cap member 142 fits. The cap 142 is shaped concavely at141 to furnish a bearing surface on which the end 140 seats to permitrocking or swiveling action of the cap with respect to the link 132. Alocking ring 143 retains the cap on the end portion 140. The hollowextension member 126 is formed with a radial extending arm 144integrally attached to the outermost extremity of said hollow extension.The arm 144 slidably receives a link or plunger 146 in a bearing 147.The link 146 is spaced radially from the link 132 and is slidableparallel to the latter link, i.e., the links 132 and 146 have relativemovement in parallel planes. The bearing 147 is provided with axiallyextending laterally opposed slots 148 in which a pin 150 reciprocates.The pin 150 is fixedat its midpoint to the link 146 at right anglesthereto so that the respective ends of the pin ride in their associatedslots for driving engagement with the arm 144. The link 146 is alsoequipped with a cap member 142 suitably attached .to the spherical end152 of said link. The two cap members 142 are drivably connected throughbearing members 154 and 156 to lever means 158 which also includes alever or foot pedal member 160 pivotally supported at 162 to a fixedpart of a vehicle, not shown, with which the boost device is used. Itwill be observed that this four bar linkage comprising lever 160, arm144 and links 132 and 146 includes a second class lever type formed bythe foot pedal member 160 which is pivoted at 162, in which theoperator's effort is applied to the pedal member at E to overcome theresistance appearing at bearing members 154 or 156, as the case may be.An inspection of the lever means 1158 reveals that the mechanicaladvantage of the lever means is greater, i.e., has a higher ratio, whenapplying a force at E to overcome the resistance offered at bearing 154than when applying a force at E to overcome the resistance offered atbearing 156. This mechanical advantage is in the relationship of theratio of the effort arm (distance from pivot 162 to point of appliedeffort E) to the resistance arm (distance from pivot 162 to hearing 154or 156). The lever means 158 establishes a lower lever ratio of pedalmovement versus piston movement with power actuation of said piston anda higher lever ratio of pedal to piston movement with manual actuationof said piston.

MODE OF OPERATION The brake booster of FIG. 1 is shown in its brakereleased position. Fluid under pressure from the pump 12 enters at theinlet port 28, and flows freely through the open center valve means 82to the exhaust port 30, from which the fluid passes through line 34 tothe steering gear 14 and thence to the return side of the pump. In thereleased position the fluid pressure levels in the booster will vary.For example, the back pressure in the cavity 54, passage 96 and annularchannel 88 is greater than that in annular channel 92, longitudinalpassage 102, radial passage 106 and chambers 42 and 136. Also, the backpressure in the annular channels 86 and 90, passages 107, 108 and 110,and cavity 52 is less than that in annular channel 92, passages 102 and106 and chambers 42 and 136. However, it is to be noted that in thebrake released position of the booster the back pressure in the chamber40 is greater than that prevailing in the chamber 42. Establishing apressure differential across the piston ends exposed to the pressures inthe chambers 40 and 42 as aforesaid assists in returning the pistonmeans 38 to its position of FIG. 1. This is achieved by the adjustingmeans 112 which trims the valve means to its desired position toestablish the pressure levels needed.

To place the booster in brake applying position with power available,the lever or pedal 160 is depressed by the operator. This pedal movementcauses the extension member 126 and spool valve member 84 to shift tothe left, as viewed in FIG. 1, thus communicating the chamber 42, on theright end of the piston to inlet port pressure via annular channel 92and longitudinal passage 102 and communicating the chamber 40 at theleft end of the piston with a back pressure less than the pressure inchamber 42 but somewhat greater than exhaust port pressure via passages98 and 100, and annular channels 94 and 90. Accordingly, a differentialpressure is created across the piston 44 acting in a manner displacingthe piston to the left so as to follow the valve spool displacement.Further pedal movement will continue to actuate the valve means 82,causing follow up of the piston means 38. It will be observed that undernormal operation of the boost mechanism 10 depressing pedal 160 movesthe link 132, extension member 126 and-spool valve 84 as a single unit,notwithstanding the fluid connection between the plunger or link 32 andthe extension member 126. This is so because the fluid in chamber 136 ispressurized, thus functioning as a solid connection between the link 132and the member 126. It will be recalled also that the area created bythe diameter of the chamber 136 is greater than the area of the annulus135 so that the net force developed in the chambers 42 and 136 acting onthe spool valve and extension member is in a direction causing movementof the spool valve to the left. The fluid pressure in the chamber 136also acts against the end of the link 132 to transmit brake feel to theoperator. With reference to FIG. 2, during a power application of theboost mechanism 10, it can be seen that link 132, extension member 126,and spool valve 84 move as a unit since the pin 138 and slot 137 are intheir same relative positions after a brake application as before.However, the pin which is fixed to the link 146 is no longer inengagement with the end of slot 148 formed in the walls of the bearing147 but has separated a distance A. Also, during the power applicationof the boost mechanism link 146 has moved a distance B and link 132 hastraveled a distance C. The distance C is equal to the sum of thedistances A plus B.

In the event of failure of fluid pressure for making a power brakeapplication, the booster is operated by depressing the lever in the samemanner as for power. In the no-power condition of the booster 10, thelink 132, attached to the lever 160 by bearing 156, moves into thechamber 136 causing separation between the pin 138, secured in theextension member 126, and the end of slot 137 in the link or piston 132,as best seen in FIG. 3. This separation is shown to be A. At the sametime the center of pivot of bearing 156 has traveled an amount B.Simultaneously therewith, the center of pivot of bearing 154, which iscloser to the fixed pivot 162 than is the center of pivot of bearing156, has moved only a distance of C. Therefore, the distance C' traveledby the center of pivot of bearing 154 is equal to the distance Btraveled by the center of pivot of bearing 156 less the distance A whichrepresents the separation between pin 138 and the left end of slot 137,as shown in FIG. 3. This relationship illustrates the high pedal ratioof the four bar linkage for no power operation of the booster. As thelever 160 is rotated about the fixed pivot 162, from the solid lineposition to the dotted line position of said lever 160, the member 126and spool 84 are shifted to the left until the centering spring 123 iscollapsed, at which time the end 121 of the valve spool drivably engagesthe inner end of the tubular member 58 contiguous said end 121. Thetubular member 58 together with the force transmitting rod 80 are nowmoved down the cylinder 18 solely by manual effort applied at E througha high ratio linkage at a relatively low effort. During this movement ofthe piston means 38 into the chamber 40 fluid is displaced throughpassages 98 and 100 into annular channel 94 where the fluid flow isdiverted into other channels and cavities of the booster mechanism andfinally to exhaust port 30.

I claim:

1. In a fluid operated boost mechanism for a vehicle, the combinationof:

housing means provided with cylinder means therein;

inlet and outlet ports in said housing in communication with saidcylinder means;

piston means reciprocable in said cylinder means;

said piston means and said cylinder means together forming two chambers,located at the respective ends of said piston means;

valve means located concentrically of said piston means for regulatingfluid flow between said ports and to said two chambers; and

operatoroperated means including valve actuating means for actuatingsaid valve means to cause movement of said piston means by fluid underpressure acting thereon and further including means for actuating saidpiston means solely by manual effort;

said operator-operated means also including lever means interconnectingsaid valve actuating means and said means for manually actuating saidpiston for establishing a low lever ratio for fluid actuation of saidpiston and a higher lever ratio for manual actuation of said piston;

said valve means having adjusting means for trimming the valve so thatwhen the operator-operated means is released fluid pressure is greaterin one of said two chambers than in the other to thereby assist inreturning said piston means to one end of said cylinder, said valvemeans is an open center valve equipped with a by-pass orifice whichsupplies a regulated but continuous flow of fluid between the inlet andoutlet ports.

2. The combination as defined in claim 1, wherein said adjustmentcomprises a valve trim screw member coaxially carried in said pistonmeans and said valve means, said screw having one end engaging saidpiston means and its other end threaded into the spool of said valvemeans, and a spring interposed between said piston means and said spoolfor retaining said spool in its adjusted position in said valve meansand for maintaining a space between said piston means and that end ofsaid spool. adjacent thereto to permit relative movement therebetween.

3. The combination as defined in claim 2, wherein said piston meansforms an abutment for engagement by said adjacent end of said spool whenthe latter moves into said space upon manual actuation of said pistonmeans.

4. In a fluid operated boost mechanism for a vehicle, the combinationof:

housing means provided with cylinder means therein;

inlet and outlet ports in said housing in communication with saidcylinder means;

piston means reciprocable in said cylinder means;

said piston means and said cylinder means together forming two chambers,located at the respective ends of said piston means;

valve means located concentrically of said piston means for regulatingfluid flow between said ports and to said two chambers; and

operator-operated means including valve actuating means for actuatingsaid valve means to cause movement of said piston means by fluid underpressure acting thereon and further including means for actuating saidpiston means solely by manual effort;

said operator-operated means also including lever means interconnectingsaid valve actuating means and said means for manually actuating saidpiston for establishing a low lever ratio for fluid actuation of saidpiston and a higher lever ratio for manual actuation of said piston;

said lever being pivotally fixed at one end and provided with a footengaging portion at its free end;

said valve actuating means including a first linkage connected to saidvalve means and to said lever; said means for manually actuating saidpiston including a second linkage connected to said valve means and tosaid lever at a point closer to the fixed pivot than said first linkageto thereby establish a mechanical advantage over said first linkage,said first linkage including a hydraulic connection.

5. In a fluid operated boost mechanism for a vehicle, the combinationof:

housing means provided with cylinder means therein;

inlet and outlet ports in said housing in communication with saidcylinder means;

piston means reciprocable in said cylinder means;

said piston means and said cylinder means together forming two chambers,located at the respective ends of said piston means;

valve means located concentrically of said piston means for regulatingfluid flow between said ports and to said two chambers; and

operator-operated means including valve actuating means for actuatingsaid valve means to cause movement of said piston means by fluid underpres sure acting thereon and further including means for actuating saidpiston means solely by manual effort;

said operator-operated means also including lever means interconnectingsaid valve actuating means and said means for manually actuating saidpiston for establishing a low lever ratio for fluid actuation of saidpiston and a higher lever ratio for manual actuation of said piston;

said means for actuating said valve means including a fluid connectionand said means for actuating said piston means solely by manual effortcomprises a mechanical connection.

6. In a fluid pressure system for a vehicle having an open center valvesteering gear in fluid flow communication with a source of fluid via aboost mechanism, said boost mechanism comprising:

housing means including cylinder means therein;

inlet and exhaust ports in said housing communicating with said cylindermeans and providing for the fluid communication with said source andsaid open center valve steering gear;

piston means reciprocable in said cylinder means;

said piston means and said cylinder means together forming two chambers,located at the respective ends of said piston means; valve meansconcentrically located with respect to said piston means for controllingfluid flow between said ports and to said two chambers; and

operator-operated means for actuating said valve means from a releasedposition to an applied position to cause movement of said piston meansunder the influence of fluid pressure and further including means foractuating said piston means solely through manual effort;

said valve means being provided with means for trimming said valve sothat the fluid pressure differential acting in said chambers is such asto assist in returning said piston means to its unapplied position uponreturn of the valve means to its released position.

7. The combination as defined in claim 6, wherein said valve meansincludes a by-pass orifice which supplies a continuous but regulatedflow of fluid to said steering gear valve.

8. The combination as defined in claim 6, wherein relief valve means isconnected between said ports to insure fluid pressure sharing betweensaid steering gear and said boost mechanism.

9. In a hydraulic power brake boost mechanism having piston means to becontrolled by valve means of an provement for adjusting said valve meansso that upon movement of the valve means to its released position, apressure differential acts across said piston means to assist inreturning said piston means, said improvement open center typepositioned coaxially of said piston 5 comprising:

and means connecting said piston means with said valve means to trim thelatter to create a higher back pressure on one end of said piston meansthan at the other end, to thereby return said piston means as aforesaid.

1. In a fluid operated boost mechanism for a vehicle, the combinationof: housing means provided with cylinder means therein; inlet and outletports in said housing in communication with said cylinder means; pistonmeans reciprocable in said cylinder means; said piston means and saidcylinder means together forming two chambers, located at the respectiveends of said piston means; valve means located concentrically of saidpiston means for regulating fluid flow between said ports and to saidtwo chambers; and operator-operated means including valve actuatingmeans for actuating said valve means to cause movement of said pistonmeans by fluid under pressure acting thereon and further including meansfor actuating said piston means solely by manual effort; saidoperator-operated means also including lever means interconnecting saidvalve actuating means and said means for manually actuating said pistonfor establishing a low lever ratio for fluid actuation of said pistonand a higher lever ratio for manual actuation of said piston; said valvemeans having adjusting means for trimming the valve so that when theoperator-operated means is released fluid pressure is greater in one ofsaid two chambers than in the other to thereby assist in returning saidpiston means to one end of said cylinder, said valve means is an opencenter valve equipped with a by-pass orifice which supplies a regulatedbut continuous flow of fluid between the inlet and outlet ports.
 2. Thecombination as defined in claim 1, wherein said adjustment comprises avalve trim screw member coaxially carried in said piston means and saidvalve means, said screw having one end engaging said piston means andits other end threaded into the spool of said valve means, and a springinterposed between said piston means and said spool for retaining saidspool in its adjusted position in said valve means and for maintaining aspace between said piston means and that end of said spool adjacentthereto to permit relative movement therebetween.
 3. The combination asdefined in claim 2, wherein said piston means forms an abutment forengagement by said adjacent end of said spool when the latter moves intosaid space upon manual actuation of said piston means.
 4. In a fluidoperated boost mechanism for a vehicle, the combination of: housingmeans provided with cylinder means therein; inlet and outlet ports insaid housing in communication with said cylinder means; piston meansreciprocable in said cylinder means; said piston means and said cylindermeans together forming two chambers, located at the respective ends ofsaid piston means; valve means located concentrically of said pistonmeans for regulating fluid flow between said ports and to said twochambers; and operator-operated means iNcluding valve actuating meansfor actuating said valve means to cause movement of said piston means byfluid under pressure acting thereon and further including means foractuating said piston means solely by manual effort; saidoperator-operated means also including lever means interconnecting saidvalve actuating means and said means for manually actuating said pistonfor establishing a low lever ratio for fluid actuation of said pistonand a higher lever ratio for manual actuation of said piston; said leverbeing pivotally fixed at one end and provided with a foot engagingportion at its free end; said valve actuating means including a firstlinkage connected to said valve means and to said lever; said means formanually actuating said piston including a second linkage connected tosaid valve means and to said lever at a point closer to the fixed pivotthan said first linkage to thereby establish a mechanical advantage oversaid first linkage, said first linkage including a hydraulic connection.5. In a fluid operated boost mechanism for a vehicle, the combinationof: housing means provided with cylinder means therein; inlet and outletports in said housing in communication with said cylinder means; pistonmeans reciprocable in said cylinder means; said piston means and saidcylinder means together forming two chambers, located at the respectiveends of said piston means; valve means located concentrically of saidpiston means for regulating fluid flow between said ports and to saidtwo chambers; and operator-operated means including valve actuatingmeans for actuating said valve means to cause movement of said pistonmeans by fluid under pressure acting thereon and further including meansfor actuating said piston means solely by manual effort; saidoperator-operated means also including lever means interconnecting saidvalve actuating means and said means for manually actuating said pistonfor establishing a low lever ratio for fluid actuation of said pistonand a higher lever ratio for manual actuation of said piston; said meansfor actuating said valve means including a fluid connection and saidmeans for actuating said piston means solely by manual effort comprisesa mechanical connection.
 6. In a fluid pressure system for a vehiclehaving an open center valve steering gear in fluid flow communicationwith a source of fluid via a boost mechanism, said boost mechanismcomprising: housing means including cylinder means therein; inlet andexhaust ports in said housing communicating with said cylinder means andproviding for the fluid communication with said source and said opencenter valve steering gear; piston means reciprocable in said cylindermeans; said piston means and said cylinder means together forming twochambers, located at the respective ends of said piston means; valvemeans concentrically located with respect to said piston means forcontrolling fluid flow between said ports and to said two chambers; andoperator-operated means for actuating said valve means from a releasedposition to an applied position to cause movement of said piston meansunder the influence of fluid pressure and further including means foractuating said piston means solely through manual effort; said valvemeans being provided with means for trimming said valve so that thefluid pressure differential acting in said chambers is such as to assistin returning said piston means to its unapplied position upon return ofthe valve means to its released position.
 7. The combination as definedin claim 6, wherein said valve means includes a by-pass orifice whichsupplies a continuous but regulated flow of fluid to said steering gearvalve.
 8. The combination as defined in claim 6, wherein relief valvemeans is connected between said ports to insure fluid pressure sharingbetween said steering gear and said boost mechanism.
 9. In a hydraulicpower brake boost mechanism haviNg piston means to be controlled byvalve means of an open center type positioned coaxially of said pistonmeans and movable from a released position to an applied position toactuate said brake boost mechanism, and a dual ratio mechanism connectedto both said means for establishing a low lever ratio for hydraulicactuation of said piston means and higher lever ratio for solely manualactuation of said piston means, an improvement for adjusting said valvemeans so that upon movement of the valve means to its released position,a pressure differential acts across said piston means to assist inreturning said piston means, said improvement comprising: and meansconnecting said piston means with said valve means to trim the latter tocreate a higher back pressure on one end of said piston means than atthe other end, to thereby return said piston means as aforesaid.